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Home » Biology Topics » Fungi      

Types of Fungi

Fungi are plant-like organisms that are parasites of either dead or living organic matter. The parasitic fungi, as opposed to the saprophytic fungi, absorb small nutrient molecules from the cells of living hosts. Many fungi are pathogenic (disease-producing) in animals and plants. Fungi contain no chlorophyll and cannot carry out photosynthesis.

Fungi are literally everywhere on Earth. They absorb their food by secreting digestive enzymes into the immediate environment. These enzymes catalyze the breakdown of large food molecules into molecules small enough to be absorbed into the fungal cell. For this reason, fungi usually grow within or on top of their food supply. This absorptive mode of nutrition is responsible for both the praises and curses earned by fungi. On the one hand, fungi are responsible for decomposing dead plants and animals, making it possible to recycle chemical elements. However, fungi also secrete enzymes that digest materials valuable to us, causing mold on food, mildew on fabrics, dry rot on wood, and athlete's foot. Commercial use of fungi and their enzymes produce wine, beer, certain cheeses, and various antibiotics.2

Characteristics of Fungi

Although some fungi are unicelullar, most are filamentous, and structures such as mushrooms consist of a great many such filaments, packed tightly together. Fungal filaments are known as hyphae, and a mass of hyphae is called a mycelium. Growth of hyphae occurs at their tips. The hyphae grow rapidly. An individual fungus may produce over half a mile of new hyphae within 24 hours. Fungi reproduce through the formation of spores that are produced either sexually or asexually. Some spores are dry and very small. They can remain suspended in the air for long periods. Other spores a slimy and stick to the bodies of insects.2

Picture of Aspergillus flavus growing on MCA

Aspergillus flavus

Picture of Aspergillus flavus; individual conidia at 1500X magnification

Aspergillus flavus (conidia at 1500X magnification)

More than 100,000 different species of fungi have been identified, and they are grouped into just four different large groups called phylia. Microscopic multicellular fungi are called molds.

1. BASIDIOMYCETES (Club Fungi)

These fungi got their name because of the tiny, "club"-shaped basidia ("little bases") growing out from the gills under the mushroom caps. The basidia are important because they produce the mushroom's spores (seeds) for reproduction. This phylum includes forms commonly known as mushrooms such as boletes, puffballs, earthstars, stinkhorns, bird's-nest fungi, jelly fungi, bracket or shelf fungi, and rust and smut fungi.

2. ASCOMYCETES (Sac Fungi)

Ascomycetes are also called "leather bag" (asco-) fungi. This group is commonly called the sac fungi because they produce spores within their sac-like or bag-like caps. This group also includes yeast. Yeast cells reproduce by smaller cells budding off and eventually separating from larger ones. Members of the genus Saccharomycetes have been used to help make bread, beer and wine since ancient times. Some common types of molds also belong to this group. A familiar example of such molds is the genus Penicillium, bluish-colored molds growing on bread, fruits, and cheeses.

Several Penicillium species produce penicillin, the powerful, bacteria-killing antibiotic drug. Another example is aspergillus causing nfections of the nervous system and lungs. Most pathogenic fungi with a known perfect state belong to this phylum.

3. ZYGOMYCETES (Conjugating Fungi)

"Zygo" literally means "yoked together." This division of fungi derives its name from the inclusion of a zygote within its life cycle. A zygote consists of two sex cells, called gametes that are literally fused, yoked, or "married" (gamet) together during fertilization. A typical zygomycetic fungus is the black bread mold, genus Rhizopus. A spore lands on a piece of white bread, then begins to germinate. The landed spore "sprouts" a number of thread-like filaments (hyphae), which soon merge to form a white, extensively branched mycelium, deep inside the bread slice. Soon, a large number of round-topped sporangia (spor for "seed", angi for "vessels") appear like tiny black puffballs. The mold is called Rhizopoda, because the sporangia and their stalks are firmly anchored into the bread surface by means of rhizoids, blunt, root-resembling or foot-resembling projections. Zygomycetes are usually saprophitic or parasitic, especially for insects. Some species may cause dangerous fungal diseases in humans and animals.

4. CHYTRIDIOMYCETES (Water-dwelling Fungi)

Chytridiomycetes are the most primitive fungi which dwell in water environments. Each chytrid has a small, globe-shaped body. It produces a highly active spore which has a flagellum (a fine, hair-like process) attached which an organism to move. In their adult stage, the chytrids make a large mycelium consisting of an extensive tangle of slender threads. These highly branched hyphae create a large area for the easy absorption of nutrients dissolved in the surrounding water. They are the only fungi that produce motile cells at some stage in their life cycle. Most are saprobes (feeding on dead or dying animals or plants) but they also contain examples of plant, animal and fungus pathogens.

Fungi, especially molds, are known to produce many tumor-inducing substances. For example, aflatoxins, potent cancer-causing agents acting mainly on the liver but also on other tissues, are produced by certain molds (mainly Aspergillus flavus) growing on peanuts. The grain-contaminating fungus, Ergot (Claviceps spp.) contains many alkaloids and other physiologically active substances, and has also been found to induce tumors in animals.

Microscope photography courtesy of Larysa Johnston

References

  1. Biology Demystified. Dr. Dale Layman
  2. Biology of Plants. Peter H. Raven, Ray F. Evert, Susan E. Eichhorn
  3. Common Poisonous Plants and Mushrooms of North America. Nancy J. Turner and Adam F. Szczawinski

 





 


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